1. Field of the Invention
The present invention relates to a method of manufacturing a high frequency electronic device, and more particularly, to a method of manufacturing a normally-off mode high frequency device structure and a method of simultaneously manufacturing a normally-on mode high frequency device structure and a normally-off mode high frequency device structure on a single substrate.
2. Description of the Related Art
High frequency electronic devices, which are operated in a high frequency region, include high electron mobility transistors (HEMTs) and metal semiconductor field effect transistors (MESFETs), and HEMTs and MESFETs are generally formed of a Group III-V compound. HEMTs are distinguished from MESFETs in that charges are transferred from a doped charge donor layer to an undoped channel layer.
Normally-off mode high frequency devices, which are also called “enhancement-mode (E-mode) high frequency devices”, are those that prevent the flow of current when no gate-source voltage is applied. On the other hand, normally-on mode high frequency devices, which are also called “depletion-mode (D-mode) high frequency devices”, are those in which current flow occurs even when no gate-source voltage is applied.
HEMTs are typically operated in a normally-on mode. In order for normally-on mode HEMTs to be changed to an OFF state, a minus power is required for applying a negative potential to a gate electrode, thus making electric circuits expensive. Furthermore, normally-on mode HEMTs have a problem of excess current flow when a power is supplied, and thus, a special circuit for preventing such excess current flow is required.
In some digital logic circuits such as DCFLs (Direct Coupled FET Logics), however, it may be preferable to simultaneously form normally-on mode high frequency devices and normally-off mode high frequency devices in a single integrated circuit.
Generally, in normally-on mode high frequency devices, a Ti/Pt/Au multilayer is used as a gate electrode. Normally-off mode high frequency devices also employ a Ti/Pt/Au multilayer as a gate electrode, but the gate electrode is not effectively embedded into an underlying barrier layer, resulting in unstable normally-off mode high frequency devices.